The present invention relates to a ceramic core for use in casting molten metallic materials having locator elements formed thereon as well as a method of forming locator elements on cores.
Most manufacturers of gas turbine engines are evaluating advanced investment cast turbine airfoils (i.e. turbine blade or vane) which include intricate air cooling channels to improve efficiency of airfoil internal cooling to permit greater engine thrust and provide satisfactory airfoil service life. Internal cooling passages are formed in the cast airfoils using one or more thin airfoil shaped ceramic cores positioned in a ceramic shell mold where the molten metal is cast in the mold about the core. After the molten metal solidifies, the mold and core are removed to leave a cast airfoil with one or more internal passages where the cores formerly resided.
The ceramic core is typically made using a plasticized ceramic compound comprising ceramic flour, organic thermosetting and/or thermoplastic binder and various additives. The ceramic compound is injection molded or transfer molded at elevated temperature in a core die or mold. When the green (unfired) core is removed from the die or mold, it typically is placed between top and bottom setters to cool to ambient temperature before core finishing and gauging operations and firing at an elevated sintering temperature.
The finished fired core is placed and accurately located in a pattern die cavity in which a wax pattern material is introduced about the core to form a core/pattern assembly for use in the well known lost wax investment casting process. In particular, the core/pattern assembly is repeatedly dipped in ceramic slurry, drained of excess slurry, stuccoed with coarse ceramic stucco or sand particles and dried to build up multiple ceramic layers that collectively form a shell mold about the assembly. The pattern then is selectively removed to leave a shell mold with the ceramic core therein.
An attempt to accurately position the ceramic core in the pattern die cavity has involved gluing plastic locators on the convex and concave airfoil surfaces of the core such that the locators will engage the wall of the pattern die cavity and positively locate the core therein. This technique is disadvantageous in that it involves a manual assembly operation that is time consuming and requires gluing. This technique also is disadvantageous in that it is subject to variations in application of the core locators on the ceramic core whereby the positions of the locators may vary from one core to the next as a result of the manual nature of the operation.
An object of the invention is to provide method and apparatus for providing locators on a ceramic core for use in casting molten metallic materials in a manner that overcomes the above disadvantages.
The present invention provides method and apparatus for forming locator elements on a ceramic core wherein a ceramic core is placed in a die cavity having a plurality of locator-forming cavities proximate the core, and a fluid material, such a melted wax, is introduced into each locator-forming cavity to form a plurality of locator elements on a surface of the core.
In one illustrative embodiment of the invention, method and apparatus for providing a plurality of locator elements on a ceramic core involves placing a ceramic core in a die cavity of die, providing a plurality of pins in the die cavity with each pin having a locator-forming cavity on an inner end proximate the core, and introducing a fluid material, such as for example melted wax, into each locator-forming cavity to form a plurality of locator elements on the surface of the core.
In a particular embodiment, each locator-forming cavity of a respective pin is communicated to a melted wax supply passage on the die for supplying the melted wax or other fluid material thereto. The die cavity includes die cavity surface regions that provide positive core location while the locator elements are being molded thereon. For example, die cavity surfaces are provided proximate the pins and configured to provide a controlled limited clearance between the core and the die cavity surfaces for positive core location and to prevent the melted wax or other fluid material from flowing between such die cavity surfaces and the core surfaces.
In an another embodiment of the invention useful for practice with an airfoil shaped core, a plurality of the pins face a concave airfoil core surface and another plurality of the pins face a convex surface core surface to form locator elements on the concave and convex airfoil core surfaces. The pins are threadably adjustable on the die to position the pins relative to different core airfoil shapes to permit the height of the locator elements to be tuned to core measurements determined to provide finished casting blueprint specifications.
The invention provides a ceramic core having a plurality of locator elements molded thereon to provide for positive location of the core in a pattern forming die.
The invention is beneficial for, although not limited to, forming locator elements on airfoil shaped ceramic cores used in the casting of gas turbine airfoils such as turbine blades and vanes especially where the core is a relatively long and thin walled ceramic core. Other objects and advantages of the invention will become more apparent from the following detailed description taken with the following drawings.